There could be a very large baryon mass in plasma in the voids.
Its emission may even have been observed: plasma
at temperature 2 x
106K fits a component of the diffuse soft X-ray background
(Wang & McCray 1993).
This might be produced locally by a relatively
small amount of gas in the Galaxy (for a summary of evidence for
and limits on hot gas around the Milky Way see
Moore & Davis 1994),
or it could be
produced by diffuse extragalactic gas with a mean density

(33)

where the emissivity parameter ranges from = 1 for solar
to = 0.1 for
primordial abundances, and the clumping parameter
C <
ne2 > / < ne
>2 may be significantly greater than unity.
Gas at this temperature at low redshift has few other detectable
effects. For example, the COBE limit on the Compton distortion
of the microwave background spectrum, y < 1.5 x 10-5
(Fixsen et al. 1996),
is produced by a Hubble length of gas at a mean
density =
(T/106)-1h-1, so the bound on
the y-distortion is not useful here. On theoretical grounds however
it is difficult to see how there could be much mass in void
plasma. If primeval, it would be surprising that the voids contain
little else in the form of baryons, especially as detected
in diffuse helium absorption at z 3 (see below). If blown in,
it would be surprising if the plasma far exceeds the density of stars, and
the required temperature in this case (to achieve sufficient
gas velocity to refill the voids in a Hubble time) exceeds
107 K, which does produce excessive anisotropy and spectral
distortion in the background radiation.